SCSI&mdash;Small Computer System Interface

The SCSI (small computer system
interface) bus is a system commonly found on workstation-class
machines, but is available for other systems such as personal
computers (PCs). The advantage of SCSI is that once the controller
is set up, you can just add new devices (typically up to seven)
with a minimum of fuss. The new device is merely plugged into the
existing “daisy chain” bus. The user must ensure that the last
device in the chain is properly terminated. We will discuss
termination in a later section.

The idea to write about SCSI came out of a project I was
working on to provide Linux-based web and network servers for a
local materials science company, Fibics, Inc.
(http://www.fibics.com/).
The network server was to include hardware for performing
file-system backups. I am not a SCSI expert and had to learn the
hard way, so I thought I'd share the basics in hopes that others
might benefit from my experience.

Where Do I Start?

You will need a computer (obviously), a SCSI controller card
to match the bus type of your computer and one or more SCSI devices
(disk drive, tape drive, etc). Note that different SCSI devices may
require different SCSI bus types (SCSI-1 vs. SCSI-2 vs. ultrawide,
etc.) and this may affect your choice of controller card. For more
information on the different SCSI buses, I recommend the booklet
Basics of SCSI from Ancot Corporation
(http://www.ancot.com/).

Before purchasing hardware, you should review the relevant
HOWTO documents (SCSI, hardware) and the hardware compatibility
list for your Linux distribution. The HOWTO documents I read are
from the Red Hat distribution and are installed in /usr/doc:
/usr/doc/HOWTO/SCSI-Programming-HOWTO and
/usr/doc/HOWTO/unmaintained/SCSI-HOWTO. The HOWTO documents provide
some useful information, although they are somewhat dated.

If your PC has no SCSI devices or controllers, you should
start by determining what device you need. For my project, a
high-capacity 4mm DAT drive was required. A review of available
drives suggested a Seagate Scorpion (12/24GB) would meet the
requirements. Once the device is selected, you can narrow down your
choice of controller cards based on the type of SCSI bus the device
requires.

The following steps were used to determine what controller
card I would require:

The target machine was an AMD K6-2-based PC with
PCI and ISA buses.

A 4mm DAT drive was needed for backup
purposes.

The selected drive required a SCSI-2 bus.

An Adaptec 2910 controller supports SCSI-2 and is a
PCI card; it is not specifically listed as incompatible in the
hardware list.

A driver for the 2910 was needed to run under Red
Hat 5.2.

The most difficult task is determining the correct driver, as
such information was not documented anywhere I looked. Seagate has
an automated phone system (1-800-SEAGATE) which provides
considerable information about their hardware. Unfortunately, on
the software side only Windows 95/98/NT information was available.

The solution was to post a message to the local Linux mailing
list. Many thanks to the individuals on the Ottawa Carleton Linux
Users Group (OCLUG) mailing list for indicating that the
“aic7xxx” SCSI driver module was the proper choice. The OCLUG URL
is listed in Resources.

Another post to the OCLUG mailing list and some looking
around led me to more detailed information. README files for the
various SCSI drivers reside in /usr/src/linux-2.0.36/drivers/scsi/.
Here, you can determine what cards or chip sets are supported by
each driver. It would be worth looking at these files prior to
purchasing a SCSI controller card.

To add the required driver, I started up Red Hat's control
panel. I selected “Kernel Configurator” and clicked on “add”. A
“Module Type” window opened; there I selected
“scsi_hostadapter” from the “Module Types” pulldown menu. I
clicked “OK” to open the “Module Definition” window, then
clicked the “Which module” pulldown and selected the required
driver. Finally, I clicked “OK” and then “Restart kerneld” in
the “Kernel Configurator” window.

Hardware

Having determined what was needed, the hardware was purchased
and installed. The Seagate DAT manual was not as clear as it could
have been about installing a single SCSI device, but all the
required information is there. Listening to the information on
their automated telephone help line answered my remaining hardware
questions. I'm providing sufficient information here (I hope), so
you won't have to make the phone call.

Two rules must be followed when dealing with a SCSI system.
One, every SCSI device must be set to a unique SCSI ID number. ID
#7 is normally used for the SCSI controller card and ID #0 is often
used by the boot drive. Two, a SCSI bus (cable) must be terminated
at both ends in this manner:

Internal devices use active termination on the last
device in the chain, at the end of the ribbon cable.

External devices require a terminator plugged into
the unused connector on the last device in the chain.

The controller card provides automatic active
termination. If the card is used only for internal devices, the
external connector need not be terminated.

In my particular case, I wanted to install a single internal
device. The 2910 controller card was plugged into a free PCI slot
on the motherboard. The DAT drive was configured for active
termination via the appropriate DIP switch. Other DIP switches were
used to set the drive to SCSI address #2. Then the drive was
installed in a free 5-1/4-inch drive bay. Since the DAT drive was
the only SCSI device being installed, it was plugged into one end
of the SCSI ribbon cable. The other end was plugged into the SCSI
controller card. The Adaptec 2910 “kit” came with the ribbon
cable that allows two devices to be connected to the card. The card
also has an external SCSI connector which was not used (nor
terminated).

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